Themodynamics and Diffusivity 101: Part II

We’re partly through our discussion of thermal diffusivity in our series on basic thermodynamics for the field thermographer. It is not our intent to have the reader become an expert in heat transfer theory. But it is important to understand the variables that effect thermal diffusivity and how this concept impacts the thermal world we may see in the field.

In The Snell Group’s Level II course we describe diffusivity with an analogy involving water, one of our favorite materials to discuss. The example is given of a volume of water in a container, and a drop of red food coloring being added. The red dot of food coloring expands as it mixes into the water. Instead of the water turning a dark red like the single drop of food color is, the water takes on a pink hue, because the food coloring disperses throughout the sample of water. It really is a great example of diffusion, and it illustrates what happens with heat energy as it diffuses through a material. You can see an example of this water experiment below with the red, representing the food coloring, gradually diffusing throughout the water within the container.

Diffusivity is essentially “spreading out” or “drawing” of the heat energy. Have you ever climbed into bed on a cool night and noticed that the covers were cool at first? As you lay there, you begin to warm up but you will notice that if you change position, and move over to a spot in the covers where you haven’t been laying, that spot is cooler. That is because your body heat hasn’t reached that area of the covers yet. When you wake up the next morning, the whole bed is warm, even spots you’ve not been laying on. That’s diffusivity at work. The heat from your body moves into the mattress, both linearly and non-linearly. At first, the greatest amount of transfer is occurring linearly from your body to the mattress material. However as you continue lying in bed, you are giving more heat energy to the mattress, and the heat begins to transfer within the material of the mattress in a non-linear fashion, with the mattress trying to reach thermal equilibrium. By the time you’ve laid there all night, the mattress is pretty uniformly warm due to the heat energy being “spread out” or diffused into the mattress material.

We’ve got one more installment on Thermal Diffusivity, and then it’s on to more Thermo!